The South route, after a gentle flat 15km opener, takes us over Aston Hill in Wendover Woods (the route shown is the direct, tougher way, but I'll put the alternative later on), past Champneys Spa and on to Tom's Hill at Aldbury (which I believe the riders went down on Colin's first trip), then a cafe stop at the Bridgewater Monument (about 45km in). I haven't been there myself, but have heard good things about it - website is
http://www.brownlowcafe.co.uk. Continues through the Ashridge Estate (National Trust), up Red Lion Hill near Hemel Hempstead, then a long gentle climb from Markyate to Whipsnade Zoo, before the sharp descent of Bison Hill. Mainly flat as we go back round the outskirts of Leighton Buzzard and to the north, where there's a final graduated climb up to Great Brickhill through Rushmere Park, then heading back to the station via the Stoke Hammond Road.
South Route map is at
https://ridewithgps.com/routes/26653905, with an easier alternative to the full Aston Hill climb to be found at
https://ridewithgps.com/routes/26872698 (the route marked in blue is about 1.5 miles longer than the original red but about the same elevation).